Intuitive Understanding of σ Delocalization in Loose and σ Localization in Tight Helical Conformations of an Oligosilane Chain. Issue 11 (26th May 2017)

• Record Type: Journal Article
• Authors: Jovanovic, Milena; Antic, Dean; Rooklin, David; Bande, Annika; Michl, Josef
• Is Part Of: Chemistry, an Asian journal. Volume 12:Issue 11(2017)
• Page Start: 1250
• Page End: 1263
• Publication Date: 2017-05-26
• Subjects: electron delocalization -- electron localization -- electronic states -- oligosilanes -- sigma conjugation; Chemistry -- Periodicals; 540.5
• DOI: 10.1002/asia.201700226
• Languages: English
• ISSNs: 1861-4728

Abstract

Abstract: Conformational effects on the σ‐electron delocalization in oligosilanes are addressed by Hartree–Fock and time‐dependent density functional theory calculations (B3LYP, 6‐311G**) at MP2 optimized geometries of permethylated uniformly helical linear oligosilanes (all‐ ω ‐Si n R2 n +2 ) up to n= 16 and for backbone dihedral angles ω =55–180°. The extent of σ delocalization is judged by the partition ratio of the highest occupied molecular orbital and is reflected in the dependence of its shape and energy and of UV absorption spectra on n . The results agree with known spectra of all‐ transoid loose‐helix conformers (all‐[±165]‐Si n Me2 n +2 ) and reveal a transition at ω ≈90° from the "σ‐delocalized" limit at ω =180° toward and close to the physically non‐realizable "σ‐localized" tight‐helix limit ω =0 with entirely different properties. The distinction is also obtained in the Hückel Ladder H and C models of σ delocalization. An easy intuitive way to understand the origin of the two contrasting limits is to first view the linear chain as two subchains with alternating primary and vicinal interactions (σ hyperconjugation), one consisting of the odd and the other of the even σ(SiSi) bonds, and then allow the two subchains to interact by geminal interactions (σ conjugation). Abstract : Why stretch the coil? The HOMO energy and UV absorption spectrum of a uniformly helically coiled oligosilane respond to chain elongation by a strong shift when the coil is extended and do not respond at all when the coil is compressed. Why are σ(SiSi) electrons in the HOMO delocalized in a loose helix and localized in a tight helix? An intuitive explanation at the Hückel level is provided.